MVC Blues - Who's In Control Here Anyway?

OK, this has me stumped...I have a Posts controller and model and a Comments controller and model. Posts has a show method and a corresponding view that shows a post and its associated comments, which are rendered with a partial from comments (<%= render :partial => "comments/comment", :collection => @post.comments %>). Another partial from comments is used to allow the user to add their comment to the page (<%= render :partial => "comments/new"%>).

Problem: If a user tries to add a new comment but it fails the comment model validation, how do I get back to the show page from posts and display the model validation error for the new comment?

controllers/comments_controller.rb ... def create   @comment = Comment.new(params[:comment])   if @comment.save     redirect_to :controller => "posts", :action => "show", :id => @comment.post   else     #we're over in the comments controller - how do we get our data validation     #errors from the comments model back over to the show page of posts?     render :controller => "posts", :action => "show" #??? end

How about "redirect_to :back" ? That sends you back to the referer.

render :action => 'posts/show' ? I'm not sure if that works though. I'd actually create create a comments/new page for this purpose.

render :action => 'new'